In motor vehicles, radar systems are used for measuring the distances and relative speeds of preceding vehicles, so that automatic speed control and distance control (ACC; Adaptive Cruise Control) can be implemented. The frequency of the microwave radiation generated by the oscillator is controlled, using the voltage applied to this oscillator. This frequency must often be modulated in the course of the radar measurements. For example, in the case of FMCW (Frequency Modulated Continuous Wave) radar, frequency is modulated using different frequency ramps, so that the differential frequency (difference in frequency) between the emitted radar signal and the radar echo becomes a function of the propagation time. The distances of objects located by the radar sensor can then be determined by evaluating the differential frequency spectrum.
For a high measurement accuracy, it is necessary that the frequencies generated by the oscillator correspond to the modulation signal with high precision. For this purpose, the frequency of the oscillator is controlled with the aid of the feedback loop and the driver in a closed control loop. In known systems, the feedback loop is a frequency-locked loop (FLL; Frequency Locked Loop). In this case, the modulation signal is a voltage signal, which indicates the setpoint frequency value valid for the specific time. The frequency of the oscillator is also converted into a voltage and compared to the modulation signal, and the driver is controlled on the basis of the comparison result.
In motor vehicles, the vehicle battery is available as a voltage source for the driver device. However, the problem with this is that this voltage source is unstable, since, depending on the charge of the battery, the battery voltage is subjected to more or less sharp fluctuations when switching other loads in the vehicle on or off. In order that the functioning of the driver device and the oscillator is not affected by such voltage fluctuations, a voltage regulator, which supplies both the driver and the feedback loop with a regulated and, therefore, stable operating voltage, has been provided, till now, between the vehicle battery and the driver.
However, the known set-up has the disadvantage that a certain, minimum voltage drop inevitably occurs at the voltage regulator, so that the DC voltage available to the driver and the oscillator is considerably lower than the battery voltage. Therefore, when the voltage of the vehicle battery is low, there may not be sufficient voltage available for operating the oscillator, and consequently, the radar system must be switched off.